The needs of acceleration of optical transmission systems have been increasingly growing with the explosive diffusion of the Internet. An optical signal of a general system is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode. This is called direct modulation. However, in this direct modulation, a high-speed modulation at several GHz or more is difficult because of influences such as relaxation oscillation. In addition, this direct modulation has a demerit such as difficulty of application to long-distance transmission since wavelength variation occurs.
As means to solve this, there is a method of using an external optical modulator. An external optical modulator performs modulation to fixed light outputted from a semiconductor laser. This is called external modulation. There is an waveguide type optical modulator, constituted by an optical waveguide formed in a substrate, as an optical modulator used for this method. Generally, the waveguide type optical modulator is small and highly efficient, and can operate in high speed.
In particular, if the optical modulator is constituted by using ferroelectric materials such as a lithium niobate (LiNbO3) crystal, it is possible to obtain a low-loss and highly efficient optical modulator. FIG. 1 shows a conventional Mach-Zehnder type optical modulator. In FIG. 1, numeral 1 denotes a lithium niobate (LiNbO3) crystal substrate. An input optical waveguide 2 is slenderly formed by making a refraction index be larger than that of the substrate by diffusing titanium on the LiNbO3 crystal substrate 1. Incidentally, a refraction index of LiNbO3 is about 2.14, but if titanium is diffused, the refraction index increases by about 0.2% to become about 2.144. Thus, a good optical waveguide is formed by using the difference of the refraction indices in a part of LiNbO3 and a part where titanium is diffused. Now, numerals 3 and 4 denote phase shift optical waveguides that each are several mm to 30 mm long and are branched from the input optical waveguide 2. An output optical waveguide 5 is connected to an outgoing side of the phase shift optical waveguides 3 and 4. These constitute a branch interference unit.
Next, an electrode section will be described. FIG. 1 shows a traveling wave type electrode section as an example of the electrode section. First, modulation electrodes 8 are formed in a pair on the optical waveguides 3 and 4 respectively. Here, the outgoing edges of the modulation electrodes 8 are terminated with a resistor R having a value near to line impedance. A modulation electric signal s(t) is inputted into incident edges of the modulation electrodes 8.
In FIG. 1, incident light 9 to the input optical waveguide 2 is divided by branching on its energy, and after passing the phase shift optical waveguides 3 and 4, is made to join in the output optical waveguide 5. At this time, if two beams of light that pass the phase shift optical waveguides 3 and 4 respectively join in the same phase, loss is small, and hence outgoing light 10 has the large quantity of light. However, if the two beams of light that pass the phase shift optical waveguides 3 and 4 respectively become in reverse phases, loss becomes large in a joint section, and hence the quantity of light of the outgoing light 10 is small. Thus, according to the amplitude of a voltage applied to the modulation electrodes 8, a refraction index of the optical waveguide 6 under the electrodes changes with an electro-optical effect. Therefore, since phases of two beams of the light which pass through it change, an optical output corresponding to the applied voltage is obtained, and hence the outgoing light 10 is modulated. In addition, in FIG. 1, the impedance of the modulation electrodes 8 is made to be near to the impedance (usually 50Ω) of an input electrical signal line. Further, the electrodes 8 are terminated with the resistor R whose value is near the impedance, and are made to be a distributed constant circuit, i.e., a traveling waveform electrode. Hence the electrodes can have a wide band.
In an optical modulator like the modulator in FIG. 1, if metal modulation electrodes 8 are directly laid on the phase shift optical waveguides 3 and 4, optical loss becomes large. Then, in order to reduce optical loss, it is effective to provide a buffer layer between the modulation electrodes 8 and the phase shift optical waveguides 3 and 4. As for the buffer layer, SiO2 is used in many cases as a raw material. It is also conceivable to constitute this buffer layer of an indium tin oxide (ITO). ITO is used in many cases as a transparent electrode, and is formed on an optical waveguide by evaporation coating. Conventionally, in this way, this electrode has three-layer structure that evaporation coating of the transparent electrode is performed on an optical waveguide and a metal electrode is further formed.
However, when the electrode section has such three-layer structure, light leaks out from the transparent electrode to the metal electrode, and in particular, under high-speed modulation, optical loss caused by leaking out cannot be disregarded.
Hence, although it is desirable to constitute an electrode only with a transparent electrode, it becomes impossible to perform wire bonding etc. without a metal electrode, and hence an optical modulator cannot be substantially constituted.